This invention relates to a semiconductor device, particularly, to a semiconductor device free from destruction even in the event of overloading.
Appended FIG. 1 shows an example of the construction of a conventional junction type transistor for power amplification, hereinafter referred to as the "power transistor", with FIG. 2 showing an example of the output characteristics of the conventional power transistor. In FIG. 1, reference numerals 1 and 2 denote a low resistive N.sup.+ region and a high resistive N.sup.- region, respectively. As shown in the drawings, a P type base region 3 is formed within the N.sup.- region 2 by selective diffusion. Further, a P.sup.+ region 4 is formed in the base region 3 for providing an ohmic contact between the base electrode and the base region. Still further, an N.sup.+ emitter region 5 is formed within the base region 3 by selective diffusion. Incidentally, the letters C, B and E shown in FIG. 1 represent a collector electrode, a base electrode and an emitter electrode, respectively.
In the graph of FIG. 2, the collector voltage V.sub.c and the collector current I.sub.c are plotted on the absissa and the ordinate, respectively, with the curves of solid lines representing the output characteristics of the conventional transistor and the broken lines indicating the secondary breakdown phenomenon. It is widely known that occurrence of the secondary breakdown phenomenon leads to the complete destruction of the transistor in many cases. Thus, in the actual operation of the power transistor, the collector voltage V.sub.c and the collector current I.sub.c are controlled not to exceed the one-dot chain line shown in FIG. 2. In general, the region beyond the one-dot chain line is called "a secondary breakdown limiting region" (S/B limiting region).
FIG. 3 shows an area of safe operation of a power transistor on the logarithmic scale. In FIG. 3, line (a) denotes the current limiting line, i.e., the allowable maximum value of current which can be conducted through the collector of the transistor. Line (b) denotes the thermal resistance limiting line. Incidentally, line (b) varies with the manner of heat releasing, ambient temperature, etc. and is also called "a power limiting line". Where the manner of heat releasing, ambient temperature, etc. are constant, the product of the voltage and current, i.e., power, is constant in any point of line (b). It follows that line (b) makes an angle of 45.degree. with each of the voltage axis and the current axis in the log-log chart of FIG. 3. Line (d) denotes the voltage limiting line. Further, line (c) denotes the S/B limiting line. Incidentally, line (c) extends in a manner to diminish the safe operation area of the transistor defined by the lines (a) (b) and (d) as well as the axes of the graph. Occurrence of the secondary breakdown phenomenon is serious in a majority of conventional power transistors, particularly, in high voltage power transistors, rendering it necessary to take some countermeasures. Specifically, it was customary in the past to use a protective circuit in operating a power transistor so that the voltage and/or the current of the transistor may not fall within the S/B limiting region.
FIG. 4 shows an example of the conventional circuit including a protective circuit. It is seen that a protective diode 7 is connected between the collector and base of a power transistor 6. In the drawing, reference numerals 8 and 10 represent a load and a fuse, respectively. Further, reference numerals 9a and 9b represent an input terminal and an output terminal to which is applied the output voltage V.sub.cc, respectively. In the circuit of FIG. 4 the breakdown voltage of the protective diode 7 is set lower than the maximum rated voltage of the power transistor 6 and higher than the operating voltage of the power transistor.
Suppose a short circuit has occurred in the load 8. In this case, the output voltage V.sub.cc is applied directly to the collector of the power transistor 6 and to the cathode of the protective diode 7. What should be noted is that breakdown occurs in the diode 7 first. Naturally, the breakdown current is added to the base current of the power transistor, resulting in that the collector current is amplified prominently because the collector current I.sub.c is h.sub.FE times as much as the base current I.sub.B in a power transistor (h.sub.FE =I.sub.C /I.sub.B). Needless to say, h.sub.FE represents the current amplification factor of the power transistor. The prominently amplified collector current causes a fuse connected to the emitter or collector (the fuse 10 connected to the emitter in the circuit of FIG. 4) to be broken, thereby preventing the power transistor 6 from being driven into the S/B limiting region. It follows that the power transistor 6 is rendered free from destruction.
FIG. 5, showing operating characteristics of a power transistor, is intended to explain the phenomenon described above. Normally, the transistor 6 operates at point (e) on the load line. But, in the event of short circuit occurrence in the load 8, the operation point of the transistor is moved to point (f) on the A--A line representing the break down voltage of the protective diode 7. If the protective diode 7 is not included in the circuit, the operation point of the transistor is moved to point (g), resulting in complete destruction of the transistor. However, the presence of the protective diode 7 permits the transistor to operate outside the S/B limiting region, leading to protection of the transistor.
If a protective circuit is not included, destruction is caused not only in the power transistor itself but also in other transistors included in the electric apparatus. In the worst case, the electric apparatus itself is completely destroyed.
As described above, it is very important to use a protective circuit in operating a power transistor. In general, zener diodes or the like are used as the element of the protective circuit. However, use of the protective circuit of this type gives rise to the following drawbacks:
1. Increase in the number of elements constituting the transistor circuit, leading to a high manufacturing cost of the transistor circuit.
2. It may happen that a protective diode having an unsuitable breakdown voltage is connected to a power transistor.
3. Much labor and a large space are required for the wiring, rendering it difficult to produce a small electric apparatus.
4. Low reliability of the transistor circuit and the electric apparatus.
5. The produced device is weak against moisture and pollution.